Drag these to your personal toolbar (Seamonkey / Firefox) or Links Bar (Internet Exploder) and click on them when needed. I have put all but the first 1 in a folder on my browser but I've kept the names short (you can rename them) so that they can go on the toolbar and still leave plenty of space for other stuff.
Disclaimer/copyright: I got the first couple of these from Jesse Ruderman. I may have made some modifications so if you want to get the original increment and decrement bookmarklets go here. The other numerical ones are derivations of his work.
Notes:
The most common use for this is to move from ...URL10.htm to ...9URL.htm as the "--" bookmarklet (and "±N" bookmarklet) preserves the overall length of the number and goes to ...URL09.htm instead.
This is a fictitious example. I can't find a useful real one right now though I have used it to navigate dates in URLs (e.g. moving from ...URL/2006/06/index.htm to ...URL/2007/06/index.htm)